Joint Resource Allocation and Reliability Maximization in NOMA-Assisted Cooperative URLLC Networks

In this paper, we focus on an ultra-reliable low latency communication (URLLC) scenario, where the access point (AP) is supposed to support latency-critical communication via a non-orthogonal multiple access (NOMA) scheme. Moreover, we allow the device with the stronger channel acting as a relay for cooperatively enhancing the transmission reliability for the other device. Based on the considered NOMA-assisted cooperative scheme, we characterize out the maximum error probability between two devices as the objective to be minimized. Together with an energy constraint for the whole transmission period, we formulate a problem jointly optimizing the blocklength assigned to two phases, i.e., the NOMA phase and the cooperative phase, and power resources allocated in each transmission attempt. To address this non-convex problem, we reformulate the problem by introducing auxiliary variables and construct a tight convex approximation at any feasible local point, based on which we further propose an efficient algorithm for iteratively improving the local point until a convergence to a sub-optimum. Via numerical results, we validate the convergence of the proposed iterative algorithm and confirm the reliability advantages of NOMA-assisted cooperative scheme, compared with multiple benchmarks.